The present invention relates to displays comprising arrays of nematic liquid crystal devices and, in particular to a system for addressing such displays.
The optical response of liquid crystal devices results from changes in the direction of orientation of the liquid crystal molecules in response to electric fields created by an applied voltage. Nematic Liquid Crystal Devices (LCD) are generally of the commonly-termed Dynamic Scattering (DS), Electrically-Controlled Birefringence (ECB) also sometimes termed Distortion of Alligned Phases (DAP), and Twisted Nematic (TN) types. DS type LCD's generate an optical response by passing a current through the device or applying an electric field to the device to create a turbulence (randomness) in the molecular orientation of the nematic medium. In ECB type LCD's the nematic medium is situated between crossed-polarizers and an optical response is generated by applying an electric field to the device to uniformly rotate in a single plane, the orientation of the molecules from an initial uniform orientation with respect to the crossed-polarizers. The TN type LCD is similar to the ECB type LCD, except that the initial orientation of the molecules is not uniform but rather comprises relatively "twisted" layers of molecules and an application of an electric field is utilized to rotate the orientation of the molecules in two-dimensions. The brightness of the response in the ECB and TM type LC is a function of the angular rotation of the molecules.
Elements of the matrix display have typically been addressed by respectively applying signals of predetermined amplitude to the row and column of the matrix in which the selected element is located, such that the composite voltage across the selected element is equal to a predetermined voltage to cause thereby a predetermined optical response (brightness). It should be appreciated, that in order to obtain proper contrast between the selected elements and the other elements in the same rows or column as the selected element, the optical response of the elements to the addressing voltages must be highly non-linear. In addition, in applications where the matrix display is scanned, for example by activating all selected elements in each successive row at a fixed rate, it may be desirable that the decay rate from an activated (on) state to an inactivated (off) state be electrically controlled. One such device having a forced erased property is a field effect device having dielectric anisotropy reversal at high frequencies such as described in an article by Baur, Steib, and Meier appearing in Applied Physics, Volume 2, 1973, page 349 et seq. Another such device is a triode optical gate liquid crystal light valve (TOG) described in " Rapid Turn-off in Triode Optical Gate Liquid-Crystal Devices" by D. S. Channin and D. E. Carlson, appearing in Applied Physics Letters, Volume 28, No. 6, Mar. 15, 1976, pg. 300 et seq.
Various schemes have been proposed for obtaining maximum contrast and eliminating cross talk between selected and non-selected elements. For example, it has been proposed that an erase signal be applied to all non-selected elements concurrently with the application of exciting signals to the selected elements. Another prior art scheme is described in U.S. Pat. No. 3,955,187 issued May 4, 1976 to J. Bigelow. In the Bigelow system a first excitation signal is applied to a selected row and a second excitation signal is simultaneously applied to all columns. Those elements having both the first and second excitation signals applied to them are thereby activated. The Bigelow patent proposes achieving zero cross talk by maintaining constant the absolute magnitude of the second excitation signal, and attaining maximum contrast by proportioning the magnitudes of the first and second signals, in a ratio dependent upon the number of columns being addressed (in the active matrix). However, as will be hereinafter more fully explained, there are well-recognized limitations on practicable matrix size for given voltage magnitudes that obtain in the Bigelow system of addressing liquid crystal displays.
The present invention concerns an addressing system that relaxes the restrictions on matrix size for given contrast ratio requirements or improves contrast ratio for fixed matrix size. In the present addressing system a predetermined exciting voltage is applied to all the elements of a selected row, to initially excite the elements in that row. Then all selected elements in the row are erased by a first predetermined amount to generate a first predetermined optical information response in the selected elements, and the non-selected elements in the row are erased by a differing amount to generate a second predetermined optical information response in the non-selected elements. Thus, one is able to take advantage of the high non-linearity of the erase characteristics of the display elements to permit larger display size for given on-off contrast ratio, to better the on-off contrast ratio for given matrix size or to otherwise obtain a more advantageous trade-off of matrix size and contrast ratio.